Proteins control all biological systems in a cell. While many proteins perform their functions independently, the vast majority of proteins interact with others for proper biological activity. The function and activity of a protein are often modulated by other proteins with which it interacts. As cells respond to myriad of stimuli, protein expression can be a transient or dynamic process. In addition, different types of cells, can express different types of proteins (cell type-dependent or -specific expression). Characterizing protein-protein interactions is important to understand protein function and the biology of the cell. While there are various methods that can be used to assess protein-protein interactions, the task can become more difficult when the proteins that interact are expressed on different cells. The methods described herein provide means for facilitating detection and quantitation of such protein-protein interactions between cells.